1. Field of the Invention
The present invention relates to a non-iridescent transparent product.
2. Discussion of the Background
Heretofore, it has been common to form a transparent conductive film on a glass surface and use it as an electrode for display or as a low-emissivity glass. For example, indium-tin oxide (ITO) formed on a glass substrate by ion plating is commonly used as a transparent electrode for a display device such as a liquid crystal device. Further, a fluorine-doped tin oxide (SnO.sub.2 :F) film formed on a glass substrate by spraying is used as a low emissivity glass for houses. Recently, as new applications of a transparent conductive film, electromagnetic wave-shielding glass for buildings, electrically-heated windshields for automobiles and transparent conductive substrates for solar cells for consumer use have been developed. In each of these applications, a glass substrate with large area is required.
As the material constituting such a transparent conductive film, ITO, SnO.sub.2 :F and aluminum-doped zinc oxide (ZnO:Al) are available. However, with these transparent oxide materials, the reflectance at a wavelength satisfying the interference conditions, tends to be high, since the refractive index is as high as from 1.6 to 2.3, which is substantially higher than glass. Namely, if such a transparent oxide is formed on a glass substrate in a film thickness exceeding a certain level, maximums and minimums appear in the spectral reflection (transmission) spectrum as ripples Accordingly, when such a film is formed on a glass substrate with a large area, the wavelength of the maximum reflection (transmission) tends to deviate due to the in-plane thickness variation (non-uniformity), which will be visually observed as iridescence of the reflected (transmitted) color.
It requires an extremely high level of technique to control the film thickness distribution on a glass surface to a level within .+-.5% by vapor deposition or CVD (chemical vapor deposition) when the glass sheet has a size of e.g. 1 m.times.1 m. According to the study conducted by the present inventors, if the film thickness distribution is at this level, in-plane iridescence reaches a problematic level when the above-mentioned transparent film is formed on a glass substrate in a thickness of at least about 0.15 .mu.m. On the other hand, when the thickness becomes at least about 0.6 .mu.m, the iridescence starts to decrease. However, the thickness is required to be at least 3 .mu.m, preferably at least 5 .mu.m, in order to reduce the iridescence to a non-recognizable level.
Even if the film thickness distribution could be controlled quite uniformly, there still remain large ripples in the spectral reflection (transmission) spectrum, and such ripples will be visually recognized as a brilliant color when the film thickness is less than 3 .mu.m. If such film is formed on a large size glass sheet, the maximum reflection (transmission) wavelength deviates depending upon the viewing angle i.e. the angle between the glass surface and the viewing direction, and the colors will thereby be changed, which will be observed as iridescence. Accordingly, it is desired to minimize ripples in the spectral reflection (transmission) spectrum.
If the film thickness is thinner than 0.15 .mu.m, the degree of the film thickness distribution is at a level of .+-.75.ANG. when a thickness distribution of .+-.5% is assumed, whereby no substantial in-plane iridescence due to the variation in the film thickness will be observed, but the iridescence due to the viewing angle will still be a problem.
Thus, when a transparent film is formed on a glass substrate in a thickness beyond a certain level, iridescence will be observed on the glass due to the in-plane thickness variation or due to the change in the viewing angle, which may substantially impair the commercial value of the product.
A few proposals have been made to prevent such iridescence. For example, a method is known wherein a transparent layer having a refractive index n=1.7 to 1.8 is formed between the transparent film and the glass substrate in a thickness corresponding to 1/4 of the designed wavelength. Further, a method is also known in which a transparent layer having a refractive index n=1.6 to 1.7 and a thickness corresponding to 1/4 of the wavelength and a transparent layer having a refractive index n=1.8 to 1.9 and a thickness corresponding to 1.4 of the wavelength are formed between the transparent thin film and the glass substrate sequentially in this order from the glass substrate side.
Further, as a practical method for forming a layer having a refractive index n=1.7 to 1.8 between a transparent film and a glass substrate, it is known to form a SiC.sub.x O.sub.y film. This is based on the float method for the production of a sheet glass and comprises impinging a gas mixture of silane, an unsaturated hydrocarbon compound and carbon dioxide to a glass surface in a molten tin bath. This is basically a method of forming a transparent film having an intermediate refractive index by normal pressure CVD.
In each case, the thickness of the underlying transparent layer is required to be a thickness corresponding to 1/4 of the designed wavelength of visible light, at the minimum. Therefore, in each case, the film forming method is normal pressure CVD. Normal pressure CVD is particularly effective from the viewpoint of low cost when a film is formed by a continuous treatment (on line) in a mass production process of glass. On the other hand, it has a drawback that it is not suitable for production of various types in small quantities or for forming multi-layers.
Therefore, in many cases, a sputtering method is employed for the production of heat reflecting glass for buildings or for automobiles. Also in the case of a transparent conductive substrate for display, it is common to employ a vacuum process from the viewpoint of the quality of the product. A vacuum process has such merits that it is readily possible to produce various types of high quality products in small quantities or to form a multi-layered film, and it is excellent in the control ability of the film thickness. Taking uniform coating to a large scale substrate into consideration, an in-line sputtering system is considered to be the best. Further, it has an additional merit that a combination with other vacuum process such as evaporation or plasma CVD can easily be designed. Therefore, it is desirable to form an underlying layer as mentioned above, by a sputtering system.
On the other hand, the sputtering method has a drawback that the film-forming speed is slow. Under the circumstances, various studies are being made to improve the film-forming speed. Especially, the film-forming speed of an oxide coating by reactive sputtering from a metal target is very slow. The present inventors previously found a transparent material with an intermediate refractive index which is stable and excellent in the durability and has a high film-forming speed, and proposed to use it as the underlying layer (Japanese Patent Application No 201148/1990) and also proposed to divide the underlying layer into two layers having low and high refractive indices (Japanese Patent Application No. 275240/1990). However, even such proposals were not entirely satisfactory from the viewpoint of the production efficiency.